1. Field of the Invention
The present invention relates to a belt apparatus and an image forming apparatus such as a copier, a facsimile machine, a printer, or other similar image forming apparatus.
2. Description of the Related Art
Recently, the market is demanding color printers and color copiers. Such color printers and copiers are conventionally either a one drum type, which includes plural developing devices positioned around a photo conductor, or a tandem type having each developing device positioned around each photo conductor. The one drum type has an advantage in that it is comparatively smaller than the tandem type and has a reduced cost. On the other hand, the tandem type has an advantage that it can provide increased copying or printing speed.
In the field of a color electronography, the tandem type device is recently used because a speed demand of the color electronography is required to be the same as with monochoromatic electronography. An example of the conventional tandem type device is explained by reference to FIGS. 9-11. FIG. 9 is a perspective view of an image forming apparatus of a direct transfer type, and FIG. 10 is a side view of the device shown in FIG. 9, while FIG. 11 is a side view of an image forming apparatus of an indirect transfer type.
As shown in FIGS. 9 and 10, photo conductors 100K, 100Y, 100C and 100M of the direct transfer type device are disposed in a straight line along a conveyance belt 500 that conveys a paper 400. Each motor 100a drives a respective photo conductor 100K, 100Y, 100C, and 100M. The conveyance belt 500 is stretched between a driven roller 501 that is driven by motor 501 a, and a roller 502. As seen in FIG. 10, the direct transfer type device includes a feed device 700 and a fixing device 800. The paper 400 is moved by the feed device 700 to the belt 500, where color toner images are formed on the paper 400 by each of the photo conductors 100K, 100Y, 100C and 100M. The paper with the toner image is then moved by the belt 500 to the fixing device 800, where the toner image is fixed to the paper.
As shown in FIG. 11, photo conductors 100K, 100Y, 100C and 100M of the indirect transfer type are disposed in a straight line along an intermediate transfer belt 600. The intermediate belt 600 is stretched around a driven roller 201, and rollers 202 and 203. Color toner images are superimposed over one another onto the intermediate transfer belt 600. A multicolor toner image on the intermediate transfer belt 600 is then transferred to a paper by way of a second transfer device 300. As with the direct transfer device, the device of FIG. 11 includes a feed device 900 and a fixing device 1000. A paper is moved by the feed device 900 to a transfer part 1100 at a nip between the roller 203 and the second fixing device 300, where the multicolor toner image is formed on the paper. The paper with the toner image is then moved to the fixing device 1000, where the toner image is fixed to the paper.
Comparing the direct transfer type with the indirect transfer type, the size of conveyance distance of the direct transfer type device is bigger than the size of the conveyance distance of the indirect transfer type device. Specifically, as seen in FIG. 10, a paper feed part 700 and a fix part 800 of the direct transfer type is required to be positioned beside the conveyance belt 500. On the other hand, the indirect transfer type device shown in FIG. 11 allows the paper feed part 900 and the fixing part 1000 to be disposed under the intermediate transfer belt 600. This positioning under the intermediate transfer belt 600 allows the conveyance distance from the feed device to the fixing device of the indirect device to be smaller than that of a direct transfer device.
Further, in order to minimize the conveyance distance of the direct transfer type device in FIGS. 9 and 10, the fixing part 800 is located very close to the conveyance belt 500. This configuration causes the fixing device 800 to influence the image formed on the paper 400 by the photo conductor 100K. On the other hand, the fixing device 1000 of the indirect transfer type device does not influence the image formation because a relatively large space is provided between the fixing part 1000 and a transfer part 1100. Therefore, the indirect transfer type device is generally preferred over the direct transfer device in terms of image quality.
In addition, both the direct and indirect transfer devices have a problem in that it is difficult to superimpose color toner images over one another in a multicolor image because the speed of the belt of these devices changes. Therefore, Japanese Registered Patent No.3186610 bulletin discloses a device that can control the speed of a photo conductor and a conveyance belt based on data obtained from detecting a pattern formed on the belt. However, when a second transfer device of the disclosed device has a change of speed, the change affects the first and the second transfers. Further, if the change is too big, speed of the intermediate transfer belt becomes out of control.